Common Mode Voltage - Help

[Jester]

Please help me get a better understanding of common mode voltage. Assuming the op amp below has a specified common mode range that is limited to the range of the power supply rails (ground to 3.3V in this case).

AND the measurement circuit on the RHS has no intentional connection to earth or chassis ground.

The voltage being measured (V1-V2) is small, however it's sitting on a large common mode voltage.

Considering that the circuit on the RHS has no intentional connection to earth (just stray capacitance), can I ignore the fact that the common mode voltage is well outside the range of U1 for this measurement?

 

[d123]

Hi,

That's an interesting question. I do not know, but isn't that chancing it a bit? That divides down to 120mV, right? You'd think that it would be okay but it looks like a dodgy method, maybe it isn't. What would happen in the case of transient voltage surges? I ask as am interested, not asking in challenging manner.

 

[crutschow]

The 6 megΩ resistance from neutral to the input and the 240kΩ resistor from input to 1.5V should establish the amp common-mode voltage within the amps range, depending upon the bias current of the op amp.

 

[KlausST]

Hi,

Without connection the opamp circuit pulls the voltage to the desired level. (About 1.5V)

But this is not a good design.

***
To make it safe, you should calculate the worst case.
120 AC means about +/-180V.
With the 1.5V offset you need a voltage divider of 1.5 : 180 = about 120.
So with 5MOhms input resistor the feedback resistor should be less than about 40kohms.

To get the desired input voltage range you need a post amplifer with gain of 6.

Klaus